Preprint Open Access

SARS-CoV-2 infections in fully vaccinated versus unvaccinated individuals

D. Zeilstra

Background. The Covid-19 pandemic is one of the most live-disturbing crises of this century. With the achievement of high vaccination coverage in high-income countries the pandemic has entered a new phase. Many countries have implemented ‘corona passports’ for traveling and/or access to specific businesses. Unvaccinated individuals can obtain such passport if they have either proof of recovery from corona or a negative test. There has been much debate about the value of these Covid certificates, in particular whether testing unvaccinated but not fully vaccinated individuals is reasonable and supported by scientific evidence. Viral loads in fully vaccinated individuals with breakthrough infections have been shown to be similar to those in unvaccinated people with a first infection. While some studies provide evidence that the odds of breakthrough infection are lower than of first infection in unvaccinated persons, these studies were conducted in specific populations. In this study data from Israel from 20 December 2020 till 4 September 2021 covering the entire general population is used to assess the vaccine effectiveness (VE) over time.

Methods. Publicly available data is retrieved from the Israeli government information site and Israeli Central Bureau of Statistics. For each week between 20 December 2020 and 4 September 2021 the VE is calculated for age groups 0-19, 20-29, 30-39, 40-49, 50-59, 60-69, 70-79, 80-89, and 90+. In addition, the VE for hospitalization and for death are calculated per week per age group.

Results. While initially the VEs in each age group are around 90%, in mid-June 2021 the VEs show a rapid decline, reaching zero or even negative values in the fourth week of June 2021 and remain close to zero until mid-August 2021. The decline of VEs coincides with the moment that the Delta variant became dominant. The absolute number of positive tests is high between mid-July and mid-August 2021, providing relative robust VE estimates. With VEs around zero in this period, this provides relative strong evidence that fully vaccinated individuals are equally at risk of infection as unvaccinated individuals. Since 15 August 2021 the VEs started to increase, coinciding with the administration of a third dose. The recovery of VEs is proportional to the percentage of people who have received the third shot per age group. While the VEs for a positive test declined and reach values around or below zero, the VEs for hospitalization and death remained relatively high over the entire period.

Conclusions. This analysis shows that in the Delta-dominated period in Israel fully vaccinated individuals are equally susceptible to infection as unvaccinated people. This does not support a regime in which only unvaccinated people are tested in order to curb infections, nor exclusion of unvaccinated individuals from activities to prevent spreading of SARS-CoV-2.The effectiveness of vaccination to prevent infections seems to be boosted by a third shot, althought here is no long term data available that enables conclusions on the duration of this effect.Reassuringly, the analysis shows that vaccination has continued to offer good protection against hospitalization and death.

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